Supplying heat to a hydrocarbon conversion vessel



Feb- 14, 19 1 c. E. JAHNIG El'AI. 2,971,823

SUPPLYING HEAT TO A HYDROCARBON CONVERSION VESSEL Filed Jan. 12, 1959 I74- FLUE GAS I WASTE HEAT 27 BOILER i 16 (TO SCRUBBER 1 26 AND COMPRESSOR) 3| 5 HYDROGEN 32 i RAINING 34 33 souos COKE HEATER 8. -20 BURNER '8 HEAT RECOVERY VESSEL 24 l9- AIR BLOWER 1 FEED AIR CRACKING PREHEAT LIFT REACTOR 4 VESSEL BUCKET LIFT lz l3 l0 8 I I HYDROCARBON AIR 9 FEED 29 J COKE I BLOWER PRODUCT Charles E. Johnig Peter L. Silveston Inventors By 3 Attorney United States atent 2,971,823 Patented Feb. 14, 1961 thee SUPPLYING HEAT TO A HYDROCARBON CONVERSION VESSEL Charles E. Jahnig, Rumson, and Peter L. Silveston, Union,

NJ., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Jan. 12, 1959, Ser. No. 786,197

7 Claims. (Cl. 23-212) The present invention relates to the production of hydrogen and by-product coke. Particularly, the present invention relates to an improved continuous process for the simultaneous production of high purity hydrogen and byproduct coke by cracking methane or other hydrocarbon gases in the presence of coke particles. More particularly, the present invention relates to a method of providing heat necessary for cracking methane or other hydrocarbon gases.

At the present time, there is an increased demand for hydrogen in many industries and this is particularly true in the petroleum industry wherein hydrogen is employed in many processes, such as the hydrodesulfun'zation of naphtha, diesel oil, fuel oil and the like. In the future, additional large quantities of hydrogen may be needed for upgrading fuel oil or for refining shale oil.

Numerous methods for converting petroleum into hydrogen have been advanced in the art. While the breakdown of methane or refinery tail gas to hydrogen and coke is a well known chemical reaction, the elevated temperatures required for desirable yields result in relatively high cost for supplying requisite thermal energy. Thus, numerous processes which might have been theoretically sound are impractical when analyzed from a commercial viewpoint.

An object of the present invention is to produce hydrogen at a relatively low cost. Another object of the present invention is to produce hydrogen and at the same time produce by-product coke of good quality. Still another object is to provide a method of heating and reheating coke which serves as a heat canier for the cracking of the hydrocarbon feed in such a manner as to minimize the burning of this valuable lay-product. These and other objects. as well as the nature and scope of the invention, will become more clearly apparent from the subsequent description and accompanying drawing.

In the present invention, coke serves as a heat carrier for cracking hydrocarbon feed and also as a surface for coke which is deposited as the feed is cracked. The advantages of using coke as a heat carrier and a depository are many and well known to the art. Coke has a high heat capacity and is therefore ideal for heat carrying. Also, during cracking operation, coke which is deposited on the surface of the heat-carrying coke particles is easily recovered by simply removing part of the heat-carrying coke particles. Coke so produced is a valuable product and is used especially in electrode manufacture. Since coke is a valuable by-product, it is advantageous to provide a hydrogen-producing or hydrocarbon cracking process which is provided with a heat recovery and preheat system to prevent the excess burning and consequent loss of this by-product.

In the present invention, the heat necessary to crack the hydrocarbon feed to hydrogen and coke is supplied to the heat-carrying coke in a burner vessel. The prior art contains numerous examples of obtaining heat transfer in moving beds, fluid beds, or a raining solids system. In these systems, the heat is obtained by passing oxygen through a bed of coke, or through falling particles of coke, and thereby partially burning the coke producing carbon monoxide and carbon dioxide, and heating the coke particles. By increasing the amount of carbon dioxide as compared to the amount of carbon monoxide, more heat is made available with less coke burning. Such control not only increases the yield of coke but also reduces the air requirement and thereby reduces the investment and operating costs.

In the usual moving or fluid beds, the contact times are relatively long. This means that the O in the air is entirely consumed and some of the CO is reduced by the coke in the bed to CO. The ratio of CO/CO is igh, so the fuel economy is poor. Shallow beds provide better fuel economy, but become inoperable. For example, in a commercial size plant producing 20 MM s.c.f./s.d. of H 22 M sci/minute of air is required in the burner, so a moving or fluid bed, if used, would have a 16 ft. internal diameter and be only 6" high.

In a raining solids burner, two operations occur simultaneously:

(1) Coke burns with air to CO and CO This step is controlled by diffusion of oxygen to the coke surface.

(2) Coke is heated to the required temperature. This is accomplished by convection and radiation from the gas to the coke surface and then by conduction into the interior of the coke particles.

In a simple raining solids burner, the operations do not overlap completely so that moretime must be provided to carry out both than would be needed for either one. As the particles in a practical size range will be accelerating throughout most of the burner, a very tall and impractical burner results. For example, with A" coke particles a -250 ft. high burner would be needed. Furthermore, extremely good air distribution of the bottom of the bed, and very even coke loading are necessary to operate this type of burner. This will be a problem at the high temperature and with changing feed rates.

In the present invention, the control problems are avoided, reasonable vessel sizes can be used, and a high CO /CO ratio is attained in a specially designed unit combining a moving bed and raining solids zone to accomplish coke heating and coke combustion separately. Thus, in the moving bed zone, only combustionis carried out. Here the coke burns with insufiicient air to carbon monoxide Whereas the raining solids zone heats the coke,

with a minimum of coke consumption. This is because the rate of heat transfer from the gas to the particles by convection and radiation is appreciably faster than mass transfer of CO to the particles surface. As only the heating operation occurs and this sets the residence time, the zone height can be minimized.

In the present invention air is admitted at the bottom of the bed and combustion takes place to carbon monoxide which leaves the top of the moving bed. Very little heating of the coke takes place here. An amount of air roughly equal to that passed through the coke bed, is introduced into the vessel just above the bed. This air burns the carbon monoxide extremely rapidly to carbon dioxide. The hot combustion gases flow up through the vessel, rapidly heating coke which is raining down. The height of the raining solids zone is critical. A minimum height is needed to bring the coke particle to the correct temperature. However, if the zone is too high, excessive coke consumption through CO reduction occurs. For this reason, the maximum height for accept able coke consumption should be no more than 10-25% over the minimum height necessary to bring the coke particles to the correct temperature. Thus, for A" coke particles, the zone must be between 30 and 50 ft. in height. The process of the present invention may be more fully understood by reference to the following description, example, and the accompanying drawing.

Referring specifically to the drawing, which diagrammatically illustrates a preferred flow plan for the practice of this invention, a hydrocarbon feed, i.e., natural gas, is fed to preheat vessel 2 where the feed is preheated to a temperature of about 1700 F. Theheat to preheat vessel 2 is supplied by a moving bed of coke particles which enter vessel 2 by means of line 3'. Coke particle size of inch provides a high surface for rapid heat exchange and permits superficial velocities as high as six feet per second of the coke particles as they pass through preheat vessel 2. By heating the feed to temperatures of about 1700 F. by the use of coke particles, some cracking takes place, but coke build-up on the A inch coke particles is balanced by the attrition of the moving bed of particles, or by supplemental attrition.

From preheat vessel 2, the preheated feed passes through line 4 and into cracking reactor 5. In reactor 5, the feed is heated to 2400 F. by contact with a moving bed of inch coke particles which enter reactor 5 through line 6. In reactor 5, cracking takes place in such a manner that all the coke is deposited on the heatcarrying coke and the gaseous products which leave reactor 5 via line 7 contain 97.5% hydrogen.

After the moving bed of coke particles has passed through reactor 5, it is withdrawn through line 8. These coke particles which are withdrawn via line S are ground to maintain the proper coke particle size. Fines from this grinding make up part of the by-product coke which leaves the system by line 9 while the rest returns to circulation by means of line From line 10, the coke particles which are at a temperature of l900 F. are lifted to storage hopper 15 by means of air lift 14. If desired, the particles may be further cooled before being lifted to storage hopper 1.5. In air lift 14, the coke particles are carried to storage hopper 15 in a dense phase by compressed air or other gas.

.The air enters the system by means of line 11 and is forced through line 13 and into air lift 14 by air blower 12. A dense phase, rather than a dispersed phase, is maintained in air lift 14, since the dense phase requires much less air and, consequently, less coke is consumed, thereby increasing the amount of by-product coke. Hopper 15 is provided to maintain the proper bed density and to smooth coke flow to burner The gas resulting from the burning of the coke is withdrawn through stack 17.

The coke particles pass from storage hopper 15 to line 16 and are transported to burner 18. Burner 18 is provided to accomplish two things: (1) increase the temperature of the coke particles from about l900 T, at which temperature the coke enters burner 13, to about 2400" F. and (2) supply the necessary heat for this purpose by burning part of the coke in such a manner as to produce combustion products having a high carbon dioxide to carbon monoxide ratio. This is accomplished by providing bed 19 below inlet line 24. Bed 19 is a moving bed or dense phase fluidized bed of coke particles about two feet deep. Air is introduced into line 21 and forced into line 23 by air blower 22. From line 23, one-half of the air so introduced passes through line 25 and into bed 19, while the other one-half of the air passes into line 24 and into burner 18 above bed 19.

Burning takes place in bed 19. By maintaining a temperature of 2400" F. in burner 18, with inch coke, a bed depth of at least one foot is sufllcient to convert the air to carbon monoxide if the velocity of the air is less than the velocity necessary to fiuidize bed 19. The air which is supplied to burner 18 above bed 19 through line 24 oxidizes the carbon monoxide passing up from the bed to carbon dioxide almost instantaneously. The air flow through lines 24 and 25 may be varied to provide a simple and efiective means of controlling coke temperature and coke consumption.

Zone 20 in burner 18 above bed 19 is a raining solids zone in which the raining coke is brought to the required temperature by convection and radiation. During the passage of the carbon dioxide through raining solids zone 20 only limited carbon dioxide reduction by the coke occurs. After passing through raining solids zone 20, the coke particles fall into bed 19 where no cooling takes place and then passes from burner 18 through line 6 to reactor 5 where the cracking takes place. The flue gases pass out of burner 18 through line 26. Waste heat boiler 27 is placed above burner 13 to recover heat from the hot flue gases which leave burner 18 at a temperature of 2400 F.

Returning now to reactor 5, the product gases containing about 97.5 percent hydrogen pass out of reactor 5 through line 7 to heat recovery vessel 28 where they contact a moving bed of coke particles. These coke particles are passed to heat recovery vessel 28 from preheat vessel 2 by means of line 29, bucket lift 30, line 31, storage hopper 32 and line 33. Storage hopper 32 is provided to smooth out the flow of coke particles to vessel 28.

The product gas enters vessel 28 at a temperature of about 2100 F. By passing through the moving bed of coke particles in vessel 28, the temperature of the product gas is decreased to about 130 F. while the coke particles in the moving bed are heated to about 1700 F. From vessel 28, the product gas is removed via line 34 to a scrubber and compressor (not shown) while the coke particles pass to feed preheat vessel 2 by way of line 3.

The conditions and requirements of burner 18-, reactor 5, preheat vessel 2 and heat recovery vessel 28 are listed in tabular form below:

BURNER 18 Preferred Range coil-e Particle Size, in.- 4 tit-1 Bod Depth, it 1-2 l-G Superfi 'al Gas V010 l 5-0 1-12 1 300 50-750 ure, psi. 3 atm.-250

le Temperature Burner. F 2. 400 2, 000-3, 000 Height of Burning Solids Zone, it 1. 30-50 10-00 CRACKING REACTOR 5 Feed Inlet Temperature, "F 1, SOD-1,800 1, 000-2, 000 Product Outlet. Temperature, T 2, 000-2, 200 l, EGO-2,000 Reactor Pressure, p.s.i.g 15-25 5-200 Superficial Velocity, I't/s 4-5 1-6 PREHEAT VESSEL 2 Feed Inlet Temperature, r 70-100 70-1, 000 Feed Outlet Temperature, T. 1, 600-1, 800 1, 000-2, 000 Superficial Velocity, this, 4-5 1-6 Vessel Pressure, p.s.i.g 10-20 awn-25D HEAT BEOOWRY VESSEL 28 Product Inlet Temperature, F 2, 000-2, 200 1, 800-2, 600 Product Outlet Temperature, 100-200 100-1, 000 Vessel Pressure, p.s '.g 20-30 atm.-200 Superficial Velocity, t., s. 6-8 1-12 1 Depends on yield: 2, 100 F-% yield; 1,800 F-92% yield (reaction becomes rate limited).

The feed for the present system may be either natural gas (methane) or refinery tail gas which has been previously treated to remove hydrogen sulfide, oxygen, carbon monoxide and carbon dioxide. These gases, if not removed, contaminate the hydrogen product. Gaseous light hydrocarbon vapors such as naphtha can also be used.

Numerous modifications may be made to the system heretofore discussed. The preheat and heat recovery system (preheat vessel, heat recovery vessel, and bucket lift) can be eliminated and an inexpensive spray tower added to the remaining equipment to cool the gaseous product before scrubbing and compression. This modification would provide only a small reduction in investment cost, since the height of the cracking reactor would have to be increased, the air blower and riser equipment would have to be larger, the burner diameter would need to be increased and fuel requirement would be increased.

Also, the preheat and heat recovery system could be eliminated and a conventional tubular gas heater which uses hot cracked gas to preheat the feed to about 1000 F. could be substituted. 1000 F. is the maximum, since this is the highest temperature possible without coke laydown. This modification would also reduce investment cost only slightly since larger equipment would be required and additional fuel would be required.

A second preheat and heat recovery system which used the heat from the burner flue gas could be installed to heat the temperature of the air used in the burner. This would increase by-product coke production and decrease air requirement but result in a much larger investment cost.

If desired, the bed of the cracking reactor could be operated as a fluid bed, since the counter-current operation of a moving bed is not especially required in the present invention.

To recapitulate briefly, the present invention relates to a continuous process for obtaining hydrogen and byproduct coke by cracking hydrocarbon gases. The present invention also relates to a particular method of heating coke particles by the utilization of a coke heater and burner which combines a moving bed and raining solids zones.

It is understood that this invention is not limited to the specific examples which have been offered merely as illustrations and that modifications may be made without departing from the spirit of the invention.

What is claimed is:

1. A method of providing heat to a reactor vessel used to convert hydrocarbon gas to hydrogen and coke which comprises maintaining a moving bed of coke particles in the lower portion of a burner vessel, passing a stream of air through the particles in the moving bed to form carbon monoxide gas, passing a separate stream of air above the moving bed of coke particles to oxidize the carbon monoxide gas passing up from the moving bed to carbon dioxide gas, introducing coke particles at the top portion of the burner vessel, showering the coke particles as raining solids through the rising carbon dioxide gas to the moving bed at the lower portion of the burner vessel, withdrawing flue gas from the top portion of the burner vessel and passing the coke particles from the moving bed to a reactor vessel.

2. The method of claim 1 in which the moving bed depth is in the range of /2 to 6 feet, the height of the burner vessel is about l0-90 feet and the coke particles size is in the range of about ,4 to 1 inch.

3. A method of cracking hydrocarbon gas to produce hydrogen and coke which comprises maintaining a moving bed of coke particles in the lower portion of a burner vessel, passing air through the particles of the moving bed to form carbon monoxide gas, passing another separate amount of air above the moving bed of coke particles to oxidize the carbon monoxide gas passing up from the moving bed to carbon dioxide gas, introducing coke particles at the top portion of the burner vessel, showering the coke particles through the rising carbon dioxide gas to the moving bed at the lower portion of the burner vessel, withdrawing flue gas from the top portion of the burner vessel and passing the coke particles from the moving bed to a reactor vessel, passing a hydrocarbon gas feed through a bed of hot coke particles in the reactor vessel, passing part of the coke particles from the reactor vessel to the upper portion of the burner vessel and withdrawing a hydrogen gas product from the reactor vessel.

4. The method of claim 3in which the hydrocarbon gas is selected from the group consisting of natural gas, naphtha and refinery tail gas from which hydrogen sulfide, oxygen, carbon monoxide and carbon dioxide have been removed.

5. The method of claim 3 in which the moving bed depth is in the range of /2 to 6 feet, the height of the burner vessel is about 10-90 feet and the coke particles size is in the range of about ,5 to 1 inch.

6. The method of claim 3 in which the hydrocarbon gas feed is heated to a maximum of 1000 F. by indirect heat exchange with the hydrogen gas product.

7. The method of claim 3 in which the hydrocarbon gas feed is preheated to a temperature in the range of about 1000" to 2000 F. by passing the hydrocarbon gas through a preheat vessel containing a moving bed of coke particles, passing the heated hydrocarbon feed preheated in the preheat vessel to the reactor vessel, withdrawing the coke particles from the preheat vessel, passing the particles to a heat recovery vessel, introducing the hydrogen gas product from the reactor vessel to the heat recovery vessel, passing the hydrogen gas product through a moving bed of coke particles in the heat recovery vessel, withdrawing a cooled hydrogen gas product from the heat recovery vessel and passing the heated coke particles to the preheat vessel.

References Cited in the file of this patent UNITED STATES PATENTS 2,389,636 Ramseyer Nov. 27, 1945 2,445,092 Utterback July 13, I948 2,448,922 Simpson et a1 Sept. 7, 1948 2,647,041 Robinson July 28, 1953 2,805,177 Krebs Sept. 3, 1957 2,885,267 Buchmann et al. May 5, 1959 

1. A METHOD OF PROVIDING HEAT TO A REACTOR VESSEL USED TO CONVERT HYDROCARBON GAS TO HYDROGEN AND COKE WHICH COMPRISES MAINTAINING A MOVING BED OF COKE PARTICLES IN THE LOWER PORTION OF A BURNER VESSEL, PASSING A STREAM OF AIR THROUGH THE PARTICLES IN THE MOVING BED TO FORM CARBON MONOXIDE GAS, PASSING A SEPARATE STREAM OF AIR ABOVE THE MOVING BED OF COKE PARTICLES TO OXIDIZE THE CARBON MONOXIDE GAS PASSING UP FROM THE MOVING BED TO CARBON DIOXIDE GAS, INTRODUCING COKE PARTICLES AT THE TOP PORTION OF THE BURNER VESSEL, SHOWERING THE COKE PARTICLES AS RAINING SOLIDS THROUGH THE RISING CARBON DIOXIDE GAS TO THE MOVING BED AT THE LOWER PORTION OF THE BURNER VESSEL, WITHDRAWING FLUE GAS FROM THE TOP PORTION OF THE BURNER VESSEL AND PASSING THE COKE PARTICLES FROM THE MOVING BED TO A REACTOR VESSEL. 